In music terms, the word "sonata" refers to a composition for an instrumental soloist, often with a piano accompaniment.

Both the original animatic of the opening credits and the doll merchandise depict Sonata wearing purple tinted sunglasses. On the doll, her blue belt also is slightly different as her belt hangs off her waist with cutie mark chains hanging off of it. Only her blue belt appears in the final film.

Depiction in Equestria Girls

In the film, Sonata and the other Dazzlings are introduced in the human world. They are sirens from Equestria who were banished to the human world by Star Swirl the Bearded before they could conquer Equestria with their dark magic. They first appear in a café causing tension between the patrons, and Sonata briefly argues with Aria Blaze. She and the Dazzlings later convince the students with their music to turn the friendly Musical Showcase into a Battle of the Bands.

Sonata's siren form projection.

Sonata and the Dazzlings use their siren's songs to control the students' opinion and advance through the competition bracket. In the final battle against the Rainbooms, Sonata and the Dazzlings are eventually defeated, and the gem pendants with which they controlled the students are destroyed. As a result, their inability to sing leads to them being chased off in disgrace.

Aside from flashbacks, Sonata's true form is never physically shown in the film, but she and her fellow band members gain pony-like ears, extended ponytails, and translucent fin-like wings. They also summon astral projections of their siren forms from their pendants.

Hasbro.com Rainbow Rocks description

Quirky and unpredictable, Sonata Dusk’s whimsical approach to everything adds a special kind of sparkle to The Dazzlings. Audiences are always enchanted by her magical voice, and her antics only turn up the volume.

Rainbow Rocks Encore description

Sonata Dusk, the member of the trouble-making band, The Dazzlings. Even though Sonata can be a bit careless sometimes, she still has a rockin’ sense of style. This fashion star is about to meet her match![3]

In the High Magic expansion set of Enterplay's collectible card game, card #33 SR of Sonata Dusk depicts her as an Earth pony and lists the quote "Over here! No wait, this way! Omigosh, is that Fighting is Magic?! I got next!"

Personality

"It's Taco Tuesday!"

Throughout Rainbow Rocks, Sonata Dusk is shown to be somewhat air-headed and carefree, similar to Pinkie Pie. For example, before the Dazzlings sing Battle in the cafeteria, Sonata is more focused on getting lunch since it is Taco Tuesday. Also, she and Pinkie are the only ones who actually seem to enjoy Snips and Snails' rap performance. Sonata doesn't pick up on Adagio's sarcasm when she says she "loves" the human world, and she speaks candidly with Sunset Shimmer about her and her friends' true nature, nearly blowing their cover. In addition, she frequently argues with Aria Blaze over trivial matters. Despite this, she can be just as cruel and devious as her bandmates, if at times unintentionally so, as evidenced in her interactions with Sunset Shimmer and Trixie.